An International Journal Effect of Calcination ...

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Effect of Calcination Temperature and Content of x and y on Crystal Structure and Microstructure of (1-xy)Bi0.5Na0.5TiO3–xBi0.5K0.5TiO3–yBiFeO3 Powders near MPB Prepared via the Combustion Technique Rattiphorn Sumang

a b

& Theerachai Bongkarn

a b

a

Department of Physics, Faculty of Science , Naresuan University , Phitsanulok , 65000 , Thailand b

Research Center for Academic Excellence in Applied Physics , Naresuan University , Phitsanulok , 65000 , Thailand Published online: 07 Dec 2013.

To cite this article: Rattiphorn Sumang & Theerachai Bongkarn (2013) Effect of Calcination Temperature and Content of x and y on Crystal Structure and Microstructure of (1-xy)Bi0.5Na0.5TiO3–xBi0.5K0.5TiO3–yBiFeO3 Powders near MPB Prepared via the Combustion Technique, Integrated Ferroelectrics: An International Journal, 148:1, 73-80 To link to this article: http://dx.doi.org/10.1080/10584587.2013.852030

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Integrated Ferroelectrics, 148:73–80, 2013 Copyright © Taylor & Francis Group, LLC ISSN: 1058-4587 print / 1607-8489 online DOI: 10.1080/10584587.2013.852030

Effect of Calcination Temperature and Content of x and y on Crystal Structure and Microstructure of (1-x-y)Bi0.5 Na0.5 TiO3 –xBi0.5 K0.5 TiO3 –yBiFeO3 Powders near MPB Prepared via the Combustion Technique RATTIPHORN SUMANG AND THEERACHAI BONGKARN* Department of Physics, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand Research Center for Academic Excellence in Applied Physics, Naresuan University, Phitsanulok, 65000, Thailand (1-x-y)Bi0.5 Na0.5 TiO3 –xBi0.5 K0.5 TiO3 –yBiFeO3 (abbreviated as BNKFT-x/y with 0.12 ≤ x ≤ 0.24, 0 ≤ y ≤ 0.07) lead-free piezoelectric powders were prepared by the combustion technique under various calcinations temperatures (600–800◦ C) for 2 h. The effect of calcination temperature and amounts of x and y on the crystal structure and microstructure were examined. The powders of all samples were well-calcined at 750 C◦ . The results indicated that there was an increase in the x and y concentrations of the crystalline structure and microstructure. XRD results of the BNKFT-x/0.03 and BNKFT-0.18/y ceramics with 0.12 ≤ x ≤ 0.24 and 0 ≤ y ≤ 0.07 showed the pure perovskite phase with a rhombohedral structure. The lattice parameter a increased from 3.8517 Å to 3.8696 Å and 3.8550 Å to 3.8623 Å with increasing x and y content, respectively. The SEM and TEM results indicated that the addition of y caused the particles to grow (from 291 nm to 308 nm) while the addition of x reduced particle growth (from 390 nm to 277 nm). Keywords (1-x-y)Bi0.5 Na0.5 TiO3 –xBi0.5 K0.5 TiO3 –yBiFeO3 ; combustion technique; crystal structure; microstructure

Introduction Bismuth sodium titanate, Bi0.5 Na0.5 TiO3 (abbreviated to BNT) has been considered to be a good candidate for lead-free piezoelectric ceramics because it has strong ferroelectricity at room temperature and a high Curie temperature T c of 320◦ C [1]. However, BNT ceramics have low piezoelectric properties, i.e. piezoelectric constant (d33 ∼58 pC/N) and an electromechanical coupling factor (kp ∼0.15). Some studies have indicated that modifications to BNT ceramic can improve its piezoelectric properties. For instance, BNT-based

Received December 9, 2012; in final form August 25, 2013. ∗ Corresponding author’s. E-mail: [email protected].

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R. Sumang and T. Bongkarn

Figure 1. XRD patterns of (a) BNKFT0.12/0.03 and (b) BNKFT0.18/0.01 powders calcined at various temperatures for 2 h: (∇ K4 Ti3 O8 ).

compositions have been modified with BaTiO3 (d33 = 132 pC/N, kp = 0.25) [2], Bi0.5 K0.5 TiO3 (d33 = 159 pC/N, kp = 0.30) [3], NaNbO3 (d33 = 88 pC/N, kp = 0.18) [4], K0.5 Na0.5 NbO3 (d33 = 90 pC/N) [5] etc. Among the solid solutions that have been developed so far the (1-x-y)Bi0.5 Na0.5 TiO3 –xBi0.5 K0.5 TiO3 (BNKT) system has attracted considerable attention. However, the piezoelectric properties of these BNKT compositions are still far from satisfactory in terms of practical applications. Recently, it has been demonstrated that a ternary system design is favorable to improve piezoelectric and ferroelectric properties. Zhou et al. [6] reported that the (1-x-y)Bi0.5 Na0.5 TiO3 –xBi0.5 K0.5 TiO3 –yBiFeO3 (BNKFT-x/y) ternary systems showed that superior electrical properties existed in the range of 0.18 ≤ x ≤ 0.21 and 0 ≤ y ≤ 0.05. The optimum values of d33 and kp are 170 pC/N and 36.6% were observed from 0.79Bi0.5 Na0.5 TiO3 –0.18Bi0.5 K0.5 TiO3 –0.03BiFeO3 ceramics. A literature survey revealed that the solid state reaction method has been used to prepare BNKFT ceramics. The pure phase was obtained by calcined and sintered temperatures above 900◦ C and 1150◦ C for 3–5 h. It is well known that this process of sample preparation is relatively simple, but it is time consuming, energy intensive and produces a poor quality ceramic. Recently, our previous works have successfully fabricated high quality different oxide ceramics such as (Ba1-x Srx )(Zrx Ti1-x )O3 [7], Ba(Ti1-x Zrx )O3 [8], (Pb1-x Bax TiO3 ) [9], 0.8Bi0.5 Na0.5 TiO3 –0.2Bi0.5 K0.5 TiO3 [10] using the combustion technique. The advantages of this technique include inexpensive precursors, a simple preparation process, and resulting high-quality powders with a low firing temperature and a short dwell time [7–11]. Furthermore, from a survey of the literature, BNKFT-x/y powders prepared by the combustion method have not been studied. Thus, in this work, (1-xy)Bi0.5 Na0.5 TiO3 –xBi0.5 K0.5 TiO3 –yBiFeO3 (0.12 ≤ x ≤ 0.24 and 0 ≤ y ≤ 0.07) powders were prepared by the combustion method. The effects of calcination temperature and the change of the x and y content on the phase formation and microstructure of powders were investigated.

BNKFT-x/y Powders near MPB Prepared via Combustion

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Table 1 Perovskite phase, lattice parameter and average particle size of BNKFT-x/0.03 powders calcined at various temperatures Composition of x


0.12

0.15

0.18

0.21

0.24

Calcination temperature (◦ C)

Perovskite phase (%)

Lattice parameter a (Å)

Average particle size (nm)

600 650 700 750 800 600 650 700 750 800 600 650 700 750 800 600 650 700 750 800 600 650 700 750 800

87.3 92.7 97.2 100 100 96.5 95.0 95.9 100 100 85.6 96.4 96.9 100 100 95.0 97.4 98.0 100 100 97.6 98.8 99.5 100 100

3.8332 3.8371 3.8376 3.8415 3.8442 3.8591 3.8597 3.8598 3.8602 3.8618 3.8562 3.8579 3.8590 3.8617 3.8628 3.8559 3.8560 3.8581 3.8658 3.8659 3.8545 3.8575 3.8624 3.8696 3.8698

264 ± 0.058 277 ± 0.088 297 ± 0.092 390 ± 0.150 411 ± 0.167 259 ± 0.085 266 ± 0.092 317 ± 0.079 296 ± 0.140 324 ± 0.084 236 ± 0.077 269 ± 0.079 271 ± 0.104 300 ± 0.096 309 ± 0.071 221 ± 0.114 230 ± 0.102 255 ± 0.096 290 ± 0.010 397 ± 0.075 210 ± 0.078 233 ± 0.096 260 ± 0.088 277 ± 0.080 283 ± 0.104

Experimental (1-x-y)Bi0.5 Na0.5 TiO3 -xBi0.5 K0.5 TiO3 -yBiFeO3 ceramics (0.12 ≤ x ≤ 0.24, fixed x = 0.03 and 0 ≤ y ≤ 0.07, fixed y = 0.18) (abbreviated as BNKFT x/0.03 and BNKFT y/0.18) were prepared by the combustion technique. The starting materials used in this study were Bi2 O3 , Na2 CO3 , K2 CO3 , TiO2 and Fe2 O3 . A stoichiometric amount of starting powders were weighed and ball-milled in ethanol for 24 h using a zirconia milling media. Drying was done at 120◦ C for 6 h. The raw material powders were well-mixed with the fuel (glycine) in an agate mortar before the calcination step. The powders were calcined between 600 and 800◦ C for 2 h. Phase identification of calcined powders was investigated in 2θ ranges of 10 to 60◦ using an x-ray diffractometer (XRD). The lattice parameter was calculated first by indexing the diffractogram according to hexagonal unit cell and the resulting parameters were then


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Figure 2. SEM images of BNKFT-0.12/0.03 calcined at (a) 600◦ C, (b) 700◦ C, (c) 800◦ C and BNKFT 0.18/0.01 calcined at (d) 600◦ C, (e) 700◦ C, (f) 800◦ C.

converted to the rhombohedral. The microstructure of all powders was observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Results and Discussion The XRD patterns of BNKFT0.12/0.03 and BNKFT0.18/0.01 calcined powders at various temperatures are shown in Fig. 1(a) and Fig. 1(b), respectively. The X-ray analysis indicated that BNKFT0.12/0.03 and BNKFT0.01/0.18 calcinated from 600◦ C to 800◦ C, had mainly a set peak with a major peak at (110). The crystal structure belonged to a rhombohedral phase, which could be matched with JCPDS file number 360340. The impurity phase of K4 Ti3 O8 was found in the BNKFT0.12/0.03 and BNKFT0.01/0.18 powders calcined below 750◦ C. Above 750◦ C the impurity phase disappeared and the sample showed a pure perovskite phase. The results of BNKFT0.15/0.03, BNKFT0.18/0.03, BNKFT0.21/0.03, BNKFT0.24/0.03, BNKFT0/0.18, BNKFT0.01/0.18, BNKFT0.03/0.18, BNKFT0.05/0.18 and BNKFT0.07/0.18 were similar to BNKFT0.12/0.03 and BNKFT0.18/0.01. The relative amounts of the perovskite phase were calculated by measuring major peak intensities of the perovskite phase. The percentage of perovskite is described by the following equation: IP erov × 100 (1) % P erovskite phase = IP erov + IK4 T i3 O8 This well-known equation is widely employed in connection with the preparation of complex perovskite structure materials [12]. Iperov and IK4 T i3 O8 are mentioned the intensity of the (110) perovskite and the intensity of the highest K4 Ti3 O8 peak. The percent of the perovskite phase of BNKFT-x/0.03 and BNKFT-0.18/y powders at various calcination temperatures was calculated and is listed in Table 1. The percentage of the perovskite phase in all samples increased with an increase of the calcination temperatures and the highest percentage was observed in powders calcined above 750◦ C. The lattice parameter a of BNKFT-x/0.03 and BNKFT-0.18/y powders at different calcined temperatures (600◦ C–800◦ C) were calculated from the (101), (012), (110), (003), (021), (202), (113), (211), (104) and (122) reflective peaks of XRD patterns and are listed in Table1. The lattice parameter a of all the samples increased with an increase of calcinations temperature. It


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Table 2 Perovskite phase, lattice parameter and average particle size of BNKFT-x/0.03 powders calcined at various temperatures Composition of y


0

0.01

0.03

0.05

0.07

Calcination temperature (◦ C)

Perovskite phase (%)

Lattice parameter a (Å)

Average particle size (nm)

600 650 700 750 800 600 650 700 750 800 600 650 700 750 800 600 650 700 750 800 600 650 700 750 800

83.5 93.2 96.2 100 100 80.8 92.7 97.2 100 100 85.6 96.4 96.9 100 100 86.1 91.8 96.2 100 100 80 89.9 96.5 100 100

3.8112 3.8276 3.8262 3.8378 3.8402 3.8208 3.8486 3.8562 3.8611 3.8694 3.8562 3.8579 3.8590 3.8617 3.8628 3.8697 3.8712 3.8864 3.8802 3.8802 3.8914 3.8864 3.9008 3.9013 3.9216

231 ± 0.084 231 ± 0.092 255 ± 0.085 285 ± 0.096 293 ± 0.088 228 ± 0.076 260 ± 0.077 291 ± 0.112 296 ± 0.092 328 ± 0.097 236 ± 0.077 269 ± 0.079 271 ± 0.104 300 ± 0.096 309 ± 0.071 216 ± 0.090 235 ± 0.114 262 ± 0.107 308 ± 0.099 309 ± 0.123 236 ± 0.088 251 ± 0.085 273 ± 0.101 310 ± 0.098 315 ± 0.058

was observed that at low calcining temperatures, the powders exist in a more strained form within the atomic entities in non-equilibrium positions, which relax to the equilibrium positions at higher temperature. It may also be that the domain mobility is restricted due to pinning of the domain boundaries by crystal defects [13]. Figure 2(a–f) shows SEM photomicrographs of BNKFT-0.12/0.03 and BNKFT0.18/0.01 powders at different temperatures. In general, the particles are agglomerated and have a spherical morphologic shape. With an increase of calcination temperatures from 600◦ C to 800◦ C, the average primary particle size increase from 264 nm to 411 nm (for BNKFT-0.12/0.03) and 228 nm to 328 nm (for BNKFT-0.18/0.01), and the agglomeration measured from 493 nm to 691nm (for BNKFT-0.12/0.03) and 467 nm to 673 nm (for BNKFT-0.18/0.01), as seen in Fig. 2(a–f) and listed in Table 1. It is also of interest to point out that average particle size tended to increase with increased calcination temperatures. This is because of the occurrence of hard agglomeration with a strong inter-particle bond within each aggregate which is the result of the firing process [14].


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Figure 3. X-ray diffraction patterns of (a) BNKFT-x/0.03 and (b) BNKFT 0.18/y calcined powders.

The SEM results of BNKFT-0.15/0.03, BNKFT-0.18/0.03, BNKFT-0.21/0.03, BNKFT0.24/0.03, BNKFT-0/0.18, BNKFT-0.03/0.18, BNKFT-0.05/0.18 and BNKFT-0.07/0.18 were similar to BNTFT-0.12/0.03 and BNKFT-0.18/0.01, as listed in Table 1. The optimum calcination temperature of all samples was found at 750◦ C for 2 h, and it was then that the amounts of x and y on the crystal structure and microstructure were examined. The XRD diffraction patterns of BNKFT-x/0.03 and BNKFT 0.18-y powders in the range of 10◦ –60◦ are shown in Fig. 3(a) and 3(b), respectively. It has been verified that all the samples are of a single-phase perovskite structure. All the peaks of the solid solution system were indexed to the rhombohedral structure and the pattern matching based on JCPDS file no. 36-0340. The position corresponding to the characteristic (110) peak shifts towards a lower angle as the x and y content are increased in the solid solution, as

Figure 4. SEM images of BNKFT-x/0.03 calcined powder with (a) x = 0.12, (b) x = 0.18, (c) x = 0.24 and BNKFT 0.18/y calcined powder with (d) y = 0, (e) y = 0.03, (f) y = 0.07.



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Figure 5. TEM images of BNKFT-x/0.03 calcined powder with (a) x = 0.12, (b) x = 0.18, (c) x = 0.24 and BNKFT 0.18/y calcined powder with (d) y = 0, (e) y = 0.03, (f) y = 0.07.

shown in Fig. 3(a) and 3(b). The lattice parameter of a increased with increasing x and y content, as listed in Table 2. It is because the higher radius (1.64 Å) of the K+ ion replaces Na+ (1.39 Å) and (0.65 Å) of the Fe3+ replaces Ti4+ (0.61 Å) resulting in the increase of the effective ionic radius of the A- and B-site respectively. The distortion of the unit cell induces strain in the lattice that may induce an increase of lattice parameter a [15]. The SEM micrographs of BNKFT-x/0.03 and BNKFT 0.18/y powders are exhibited in Fig. 4(a–c) and 4(d–f), respectively. All the particles are spherical in shape and agglomerated. With an increase of x content from 0.12 to 0.24, the average particle size decreases from 390 nm to 277 nm (Table 2). As the y content increased, the average particle size increased from 285 nm to 310 nm (Table 2). The TEM micrographs of BNKFT-x/0.03 and BNKFT 0.18/y powders are shown in Fig. 5(a–c) and 5(d–f), respectively. It can be seen that the powder particles have a similar spherical shape and a porous agglomerated form. With an increase of the x content from 0.12 to 0.24, the average particle size decreased from 197 nm to 92 nm. The average particle size increased from 189 nm to 244 nm with an increase of the y content. However, the TEM results were different in their values when compared with the particle size from the SEM image. This may have been caused by the agglomeration affects in the SEM results.

Conclusions BNKFT powders were synthesized by the combustion technique. The optimal calcinations conditions were found to be 750◦ C for 2 h in all the conditions. Calcinations temperatures and the content of x and y have a strong influence on the crystal structure, microstructure, lattice parameter and percentage of the perovskite phase of the calcined powders. The lattice parameter of a increased with increasing x and y content. The ceramics possess a pure single phase of perovskite structure, indicating that K+ and Fe+ have diffused into the lattice. The SEM images indicated that with increasing x content, particle size decreases. But with

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increasing y content, the variation of the particle size is opposite. BNKFT powders can be successfully synthesized by the combustion technique at a lower calcination temperature than those prepared by the solid-state reaction method.


Acknowledgments This work was financially supported by the Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant no. PHD/0213/2552) to Miss Rattiphorn Sumang and Assist. Prof. Dr. Theerachai Bongkarn. The authors wish to thank the Naresuan University for supporting facilities. Thanks are given to Mr. Don Hindle for his help in editing the manuscript.

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